Recently, with spread use of IT (Information Technology) equipments, an amount of information traffic in an information processing apparatus is increasing enormously. Hence, a signal bandwidth in the information processing apparatus is also increasing. A through-hole stub does not exert no effect on a transmission characteristic as long as a signal propagation is propagated at a rate of the order of 1 to 6 Gbps (Gigabits per second). However, when a transmission rate surpasses 10 Gbps, deterioration in the transmission characteristic begins to be markedly noticeable.
Among the related techniques to overcome the deterioration in a characteristic caused by a through-hole stub, there is a technique of through-hole processing by back-drill that cuts a through-hole and a near-by board portion using a drill. The through-hole portion is cut by a drill having a diameter slightly larger than an external shape of the through-hole to bore a hole to remove a through-hole portion corresponding to a stub. However, only few board manufacturers are able to use a back-drill at their disposal. Moreover, the back-drill is of a problem in connection with cost and supply. There is thus a demand for a technique which should take the place of back-drilling. The following describes a typical example of a backplane system as a transmission system.
FIG. 1 is a diagram illustrating an example (prototype) of a backplane system used in a communication equipment, as an example. A backplane is a sort of a printed circuit board, and includes, on its lateral side, a plurality of connectors, also termed backplane connectors. A plurality of cards, mounted on the connectors, are interconnected to form a bus system. Note that a midplane includes a plurality of connectors (backplane connectors) on both sides of its circuit board. Although in the following description, a backplane structure is described as an example, it is possible to replace the backplane structure with a midplane structure.
Referring to FIG. 1, line cards 11 and a switch card 12 are mounted on connectors of a backplane 14 (backplane connectors 13). The backplane takes charge of electrical connection between the switch card 12 and one of the line cards 11 to perform signal transmission of a line signal via the switch card 12 to the other line card 11. Recently, a line speed transitions from 1 Gbps to 10 Gbps, and is going to evolve further to 40 Gbps or even to 100 Gbps. It is thus necessary to speed up backplane transmission with deterioration of a signal characteristic being suppressed, as shown in FIG. 1.
FIGS. 2A and 2B illustrate a configuration (physical specifications) of the backplane system shown in FIG. 1. FIG. 2A is a diagram schematically illustrating a lateral cross-sectional view of the backplane, connectors and the cards (boards). FIG. 2B is a diagram schematically illustrating sectional view of areas encirclued by dotted-line circles of the board and the backplane of FIG. 2A, and illustrates a through-hole and a stub.
Referring to FIG. 2A, a line signal is coupled on a path including an IC (Integrated Circuit) 22A-> a board 21A (a small-sized printed circuit board mounted on the backplane, also termed a ‘daughter card’ or ‘a daughter board’)-> a connector 23A-> a backplane 24-> a connector 23B, -> a board (daughter card) 21B-> an IC 22B. The connector 23A (23B) includes a terminal (connector terminal) inserted (press-fit) into a through-hole of a board (daughter card) 21A (21B) and another terminal (connector terminal) inserted (press-fit) into a through-hole of the board of the backplane 24.
FIG.2B schematically illustrates a cross-section of a multilayer board. A signal wiring (interconnect) is coupled to a signal layer (signal) at a preset depth (the depth corresponding to a depth of the signal layer from the board surface) from a through-hole surface coated with an electrically conductive member such as plating to establish electrical conduction. That is, an electrical signal fed to the signal layer (signal), is supplied from an upper part of the through-hole to enter into the signal layer (signal) near at a midpoint of the through-hole, as shown in FIG.2B. Since the through-hole extends to underneath the portion at which the signal layer (signal) is connected to the through-hole, the portion at which the signal layer (signal) is connected to the through-hole (a bending potion of the signal wiring in FIG.2B) becomes a branch point of signal path. Hence, the signal propagated from the upper part of the through-hole is propagated at the branch point (bending portion) into the signal layer (signal) in the multilayer board. However, part of the signal propagated from the upper part of the through-hole proceeds from the branch point further downward through the through-hole. The part of the through-hole underneath the branch point, though in itself not being a signal path, is electrically conductive and hence becomes a signal propagation path. In the case where a signal path is branched at a branch point in this manner, the part which is not in itself a signal path is generally termed a “stub” (stub: branch wiring). The signal proceeding from the branch point downward through the through-hole is reflected back at a bottom end portion of the through-hole and proceeds upwards through the through-hole to return to the branch point. There are times when the signal proceeding downwards from the branch point collides against the signal reflected back from the bottom end portion of the through-hole to affect adversely a transmission characteristic of the signal. The effect is outstanding in a high frequency signal, a high speed digital signal and the like. In FIG.2B, on a lower-side ground plane (power supply) area surrounding differential via (via-hole), there is formed an opening (clearance) freed of the ground plane (power supply), and is termed an anti-pad.
FIG. 3 is a diagram illustrating an example of a connection configuration between a connector (backplane connector 33) and a board (daughter card) and between the connector (backplane connector 33) and a backplane. As the backplane connector 33, a press-fit connector in which its connector terminals 35 are press-fit into corresponding through-holes 34 formed in the board 3, may be used but not limited thereto.
FIG. 4 is a diagram illustrating a signal propagation through a connection portion between the connector (backplane connector) of FIG. 3 and a board (a daughter card or a backplane). A board 41 of the daughter card or the backplane is a multilayer board including a power supply layer or a GND layer (ground layer or ground plane) 42, a signal layer 44 and a dielectric material 43 between the respective layers. The signal layer 44 is provided between GND layers 42, for example. Referring to FIG. 4, a signal from a connector terminal 45 (corresponding to the connector terminal 35 of the backplane connector 33 of FIG. 3) at a top end portion of a through-hole 46, is propagated into the signal layer 44 at a branch point of the signal layer. However, part of the signal flows, within the terminal 45, from the branch point to a downward portion of the through-hole 46 to be reflected back at the bottom end of the through-hole 46. The so reflected signal part collides, at the branch point to the signal layer 44, against the signal proceeding via the upper part of the connector terminal 45. That is, the reflected wave encounters further reflection at the branch point within the through-hole where there occurs multi-reflection. The end part of the connector terminal 45 in the through-hole 46 is open and hence the signal undergoes total reflection. The branch point in the through-hole 46 (point of connection to the signal layer 44) is at a low impedance, and hence the signal is reflected with phase inversion. As a consequence, there results a quarter-wavelength resonance by a standing wave having the end part of the connector terminal as an anti-node and the branch point in the through-hole as a node.
With a stub length L (in FIG. 4, a length between the coupling part of the signal layer 44 to the through-hole 46 and the lower end of the through-hole), the wavelength γ of the standing wave is given by the following equation (1):λ=4L/n (n=1, 3, 5, . . . )  (1)
The product of the resonance frequency f and the wavelength γ is the velocity of light, such that
                                                                                          f                  ×                  λ                                =                                ⁢                C                                                                                                          =                                    ⁢                                      C                    ⁢                                                                                  ⁢                                          O                      /                                                                                                                                                              ⁢                                          (                                              ɛ                        ⁢                                                                                                  ⁢                        r                                            )                                                                      ⁢                                                                                                      ⁢                                  ⁢                  (                      C            ⁢                                                  ⁢            being            ⁢                                                  ⁢            the            ⁢                                                  ⁢            velocity            ⁢                                                  ⁢            of            ⁢                                                  ⁢            light            ⁢                                                  ⁢            through            ⁢                                                  ⁢            a            ⁢                                                  ⁢            substance            ⁢                                                  ⁢            having            ⁢                                                  ⁢            a            ⁢                                                  ⁢            specific            ⁢                                                  ⁢            inductive            ⁢                                                  ⁢            capacity            ⁢                                                  ⁢            ɛ            ⁢                                                  ⁢            r                    )                                    (        2        )            where CO is the velocity of light in vacuum, and is given byCO=1/√{square root over ( )}(εO×μO)  (3)where εO and μO stand for the specific inductive capacity and the magnetic permeability of vacuum, respectively.
Hence, the resonance frequency f is given by the following equation (4):f=n×CO/(4×L×√{square root over ( )}εr)  (4)
In equation (4), n is a positive odd number (1, 3, 5, . . . ), CO is the velocity of light in vacuum, L is the stub length and εr is the specific inductive capacity.
FIG. 5 is a diagram explaining the above mentioned signal transmission path (differential transmission path). In FIG. 5, backplane connectors 54A and 54B correspond to the connectors 23A, and 23B of FIG. 2A, respectively. In FIG. 5, a connector terminal 53A corresponds to a connector terminal of the connector 23A connected to the through-hole of the daughter card 21A of FIG. 2A. and a connector terminal 55A corresponds to a connector terminal of the connector 23A connected to the through-hole of the backplane 24 of FIG. 2A. In FIG. 5, a connector terminal 55B corresponds to a connector terminal of the connector 23B connected to the through-hole of the backplane 24 of FIG. 2A. A connector terminal 53B corresponds to a connector terminal of the connector 23B connected to the through-hole of the daughter card 21B of FIG. 2A.
A signal differentially output from an output buffer 51 (an output buffer, not shown, in the IC 22A of FIG. 2A) is delivered to the connector terminals 53A of the backplane connector 54A via wirings 52A in a daughter card (corresponding to the board 21A of FIG. 2A, for example), the connector terminals 55A of the backplane connector 54A, wirings 56 in the backplane (corresponding to the signal layer 44 of FIG. 4, for example), the connector terminals 55B of the backplane connector 54B, the connector terminals 53B of the backplane connector 54B and via wirings 52B in a daughter card (corresponding to the board 21B of FIG. 2A, for example) differentially to an input buffer 57 (an input buffer, not shown, provided in the IC 22B of FIG. 2A). The input buffer 57 includes a termination resistor between differential inputs. The differential input signal is supplied to an equalizer circuit and equalized.
A signal received from a connector via a wiring is deteriorated in a manner as detailed with reference to FIG. 4, under the effect of a reflected wave generated at an open end of a stub parasitically produced within a through-hole provided in a backplane. There occurs energy distribution into an energy passing from the branch point in the through-hole into the through-hole and an energy passing through a board (a signal layer in a daughter card or in a backplane). The energy passing through the through-hole is reflected at an end of the through-hole (open stub end). There occurs at the branch point in the through-hole further reflection of the reflected wave, and here multi-reflection occurs. Hence, a quarter-wavelength resonance occurs by a standing wave having an end of the connector terminal as an anti-node and the branch point in the through-hole as a node.
An insertion loss in a differential through-hole of FIG. 7 shows the results of an analysis of a through-hole structure (differential through-hole) of FIG. 6, in order to demonstrate above mentioned phenomena. In FIG. 7, the abscissa is a frequency and the ordinate is an input differential insertion loss Sdd21 (unit in dB). In the example of FIG. 7, the input differential insertion loss Sdd21 is of about −24 dB, in the vicinity of 7 GHz (maximum absolute value of the insertion loss (attenuation)), due to the quarter-wavelength resonance in the through-hole stub.
In FIG. 6, a backplane connector terminal pair 67, differentially transmitting a signal, is connected to a signal through-hole pair 62. The signal through-hole pair 62 is connected, in the signal layer disposed between GND layers 64, to a signal wiring pair 65. In FIG. 6, a stub (through-hole stub) 66 is a section between an open end at a bottom end of the signal through-hole 62 and a connection portion of the signal through-hole 62 and the signal wiring 65.
With a speed up of a line interface, a transmission rate not less than 10 Gbps is required on the backplane. Due to the through-hole characteristic described above, it may be understood that transmission is difficult.
Several literatures to solve this problem are known. However, they have respectively certain drawbacks.
Patent Literature 1 discloses a circuit board in which at least part of a through-hole and a via is drilled to reduce length of an electrically conductive stub of the hole, wherein the drilled part of the hole includes a transitioning portion from a first profile to a second profile to reduce reflection from the drilled hole end portion. The technique disclosed cuts the stub of the through-hole by drilling to reduce resonance caused by the stub. The technique is good in characteristic. However the drilling is difficult to control during board fabrication and there is concern about high costs due to the problem of yield or the like. Moreover, quality-related problems caused by residual burrs in cutting process by drilling have not yet been solved.
To address a problem that in high speed transmission of a signal on a differential wiring, waveform distortion occurs due to impedance mismatch, in a via-hole with an open stub, thus producing the jitter, Patent Literature 2 discloses a configuration in which the degree of coupling of the differential wiring is reduced, with a differential characteristic impedance remaining constant. That is, there is disclosed a technique in which the degree of coupling of the differential wiring is optimized to reduce an adverse effect caused by the through-hole stub. The technique presupposes that deterioration by a stub occurs at a frequency range sufficiently higher than an operating range and that the signal in the frequency range lower than the operating range is to be stabilized. In short, the technique disclosed is not able to compensate for the deterioration characteristic proper to the stub. There is not disclosed such a technique that overcomes limitations in the case where the signal frequency range is approximately the same as the frequency range of deterioration produced by the stub.
Patent Literature 3 discloses a method which optimizes a via structure to improve a high frequency performance of a backplane, and a method which optimizes a size as well as a shape of the via structure to improve its high frequency integrity performance. In FIG. 2 of the Patent Literature 3, an electrically conductive portion of a via composing a stub section is removed by drilling to remove a non-used stub portion of a plated through-hole (PTH). This leads to a problem of complicated designing. Moreover, problems of cost as well as quality due to back-drilling are not solved.
Patent Literature 4 discloses a configuration in which an integrated circuit includes an enclosed termination resistor designed to match to a characteristic impedance of a transmission line (signal source impedance), and in which the signal source drives a plurality of IC devices on a printed circuit board. The IC devices are cascade-connected in a chain, and internal resistors of the IC devices except the last IC device in the chain are bypassed by a short circuit underneath the IC devices, with the internal resistor of the last IC device in the chain not having a short circuit underneath it. Although a technique of providing the termination resistor within the IC device to increase the bus speed is disclosed, the technique does not solve the problem of deterioration in the through-hole characteristic brought about by a stub.